One form of heating element composition known in the art comprises a mixture of rubber or other elastomeric polymer with conductive particles, such as carbon black and graphite. Such a composition is conventionally known as a thermal composition, and one example thereof is shown in the Japanese Patent Laid Open No. 75705/1983 or No. 8443/1981.
Such thermal compositions are commonly formed of rubber. The composition has a positive temperature coefficient with respect to the heating effect of electrical current passed therethrough. Thus, as the temperature of the composition rises as a result of the current flow through the limitedly conductive material, the positive temperature coefficient thereof causes an increase in the resistance so as to reduce the current. The equilibrium point is reached wherein the current is maintained at a value suitable to produce heat in the body of the composition at a rate equal to the rate at which the heat is dissipated from the surface thereof. Illustrative uses of such heating elements are for melting snow on roofs and the like, preventing of freezing of pipes and road surface areas, etc.
The conventional thermal compositions, however, have a number of serious deficiencies and have not proven completely satisfactory heterofore. Illustratively, because of the relatively small positive temperature coefficient, substantial time is required to arrive at the stable predetermined temperature. The known thermal compositions, further, are relatively unstable and have irregular thermal characteristics, resulting in reduced efficiency and increased poser consumption. Changes in the operating characteristics of the compositions occur because of the thermal expansion and degradation of the composition caused by uneven and excessive heating of different portions thereof.